One of the problems associated with the pressure tubes of a CANDU (CANadian Deuterium moderated Uranium) fuel reactor is the risk of delalyed hydrogen cracking (DHC) of the material due to the presence of hydrogen and stress. The term "hydrogen" as used herein includes the isotopes of hydrogen, such as deuterium. A cracking problem occurs generally in pressure vessels containing hydrogen. In present CANDU reactors, pressure tubes are mointored for the rate of hydrogen ingress, and evaluated for the risk of DHC. If the risk becomes too high, or if the tube leaks, they are replaced. The monitoring and replacement of pressure tubes involves considerable time and expense. for non-nuclear applications, pressure vessels for containing hydrogen have been proposed that use an inner layer that has reduced susceptibility to embrittlement by hydrogen but does not prevent the migration of hydrogen which is vented through openings in the outer layer. Such multi-layer vessels are made with a shrink or interference fit that produces compressive forces in the inner layer. These shrink fit type vessels would not be suitable when subjected to a neutron flux. Such environment produces radiation induced creep and growth, hereinafter referred to as deformation, of the load bearing layer and results in tensile stresses in the inner layer which may lead to delayed hydrogen cracking (DHC) of th inner sealing layer. A crack in the sealing layer may permit high local concentrations of hydrogen in the underlying load bearing layer and may lead to non-ductile failure of the vessel.
In the case of pressure tubes for a nuclear reactor, the design is made more difficult by the desirability of using material with a low neutron absorption cross section, and good in reactor corrosion resistance.